1. Field of the Invention
This invention relates to an improved recycle vapor-liquid separator in an ebullated bed process. The separator comprises a cup with a plurality of riser conduits and two stages of cyclone separators.
2. Description of Other Relevant Methods in the Field
The ebullated bed process comprises the passing of concurrently flowing streams of liquids or slurries of liquids and solids and gas through a vertically cylindrical vessel containing catalyst. The catalyst is placed in random motion in the liquid and has a gross volume dispersed through the liquid medium greater than the volume of the mass when stationary. This technology has found commercial application in the upgrading of heavy liquid hydrocarbons or converting coal to synthetic oils. The process is generally described in U.S. Pat. No. 25,770 to Johanson incorporated herein by reference. A mixture of hydrocarbon liquid and hydrogen is passed upwardly through a bed of catalyst particles at a rate such that the particles are forced into random motion as the liquid and gas pass upwardly through the bed. The catalyst bed motion is controlled by a recycle liquid flow so that at steady state, the bulk of the catalyst does not rise above a definable level in the reactor. Vapors along with the liquid which is being hydrogenated pass through the upper level of catalyst particles into a substantially catalyst free zone and are removed at the upper portion of the reactor.
In an ebullated bed process the substantial amounts of hydrogen gas and light hydrocarbon vapors present rise through the reaction zone into the catalyst free zone. Liquid is both recycled to the bottom of the reactor and removed from the reactor as product from this catalyst free zone. Vapor is separated from the liquid recycle stream before being passed through the recycle conduit to the recycle pump suction. The recycle pump (ebullation pump) maintains the expansion (ebullation) and random motion of catalyst particles at a constant and stable level. Gases or vapors present in the recycled liquid materially decrease the capacity of the recycle pump as well as reduce the liquid residence time in the reactor and limit hydrogen partial pressure.
Reactors employed in a catalytic hydrogenation process with an ebullated bed of catalyst particles are designed with a central vertical recycle conduit which serves as the downcomer for recycling liquid from the catalyst free zone above the ebullated catalyst bed to the suction of a recycle pump to recirculate the liquid through the catalytic reaction zone. The recycling of liquid from the upper portion of the reactor serves to ebullate the catalyst bed, maintain temperature uniformity through the reactor and stabilize the catalyst bed.
U.S. Pat. No. 4,221,653 to Chervenak et al. describes an apparatus for separating vapor from liquid in an ebullated bed process. The apparatus comprises a frusto-conical cup in which are inserted a plurality of riser conduits. The conduits are positioned in two concentric circles within the cup. The generic term for the recycle gas-liquid separator apparatus in an ebullating bed process is a recycle cup. The recycle cup of the Chervenak et al. patent and those like it with a plurality of riser conduits are referred to as a tubular recycle cup.
It is a critical feature of the recycle cup that the upflowing liquid-gas mixture rising from the reaction zone passes through the riser conduits of the separation apparatus and that lower ends of all conduits are below the reactor liquid level. After passage through the recycle cup, the gas portion rises to the top of the reactor. Part of the liquid portion is returned through a downcomer conduit and recycled to the reaction zone. The remaining liquid portion is withdrawn from the reactor as liquid product. The returned liquid portion passes through the recycle conduit to a recycle pump, then passes through a liquid-gas distribution means, together with fresh liquid and hydrogen feed to maintain uniform upward fluid flow through the ebullated catalyst bed. The liquid and vapor effluent may be withdrawn separately from the upper portion of the reactor. If withdrawn separately, a second interface between liquid and vapor is established. Vapor is withdrawn from above the interface. The liquid is withdrawn from a point in the reactor free of vapor. If desired, liquid and vapor portions may be withdrawn together through a single conduit extending into the reactor to a position adjacent the separator apparatus.
U.S. Pat. No. 4,151,073 to A. G. Comolli and U.S. Pat. No. 4,354,852 to P. H. Kydd recognize the advantages of effecting the recycle liquid-vapor separation in an ebullated bed process by feeding the fluid tangentially to a cylindrical separator. By this method, the hot fluid is fed to the cylindrical separator at conditions to prevent carbonaceous particulate material from depositing on the interior surface of the separator. These conditions include tangential injection of feed to the separator, fluid temperature of 550.degree. F. to 900.degree. F. and a separator length/diameter ratio of 20/1 to 50/1. The Kydd patent additionally teaches that a liquid vortex in the cylindrical separator reduces coke deposition.
The design of liquid cyclone separators is well known in the art. For example, U.S. Pat. Nos. 3,668,116 and 4,012,314 describe the use of a liquid cyclone in an ebullated bed process. An essential feature of any cyclone is tangential feed to a circumferential wall.
U.S. Pat. No. 4,886,644 to T. Y. Chan et al. teaches a liquid degasser in an ebullated bed process. The degasser comprises a cup with a plurality or riser conduits. Helical members are positioned in the riser conduits and cyclone separators in fluid communication therewith.